I'm thinking that they'll have a pool of Pokemon to choose from, and three will be randomly chosen each time you battle, just so you don't battle the same three each time... that would get boring. I'd hope that they'd be creative in their teams and movesets so you can't just go in with a supereffective Pokemon and sweep them. Using Blaine as an example, he could equip a Typhlosion with Thunderpunch or something, or use a Clefable with Fire Blast.
Right now it's just Champions and Leaders, but there's always some possibility of extending the roster to include others at some point in the future. If they, Leaf would be my vote - they've used her male counterpart in way too many games thus far. She needs love Again, they may limit it to Leader and Champions and not include others at all; it's still too early to call. GF has been pulling some pretty crafty tricks with these games; nothing would surprise me at this point.
And I've said this before, but I'd really rather see the Unova Gym Leaders have a rematch system separate from the tournament. I'm still wondering as well what Akuroma's role in all this is because he's not a Champion or a Leader, and he's not a character from a past game.
Still not sure if I'll like this, as the Battle Tower/Frontier was always a source of rage and frustration for me due to the hax luck, but I'll say it's an interesting idea.